In principal, dissimilar conductive particles can be separated electrostatically by a variety of methods that are well documented in the literature. One type of electrostatic separation method that has achieved the greatest commercial success utilizes a triboelectric counter-current belt-type separator as disclosed in U.S. Pat. Nos. 4,839,032 and 4,874,507. Such belt separator systems separate the constituents of particle mixtures based upon the charging properties of the different constituents by surface contact, i.e. the triboelectric effect. These systems typically utilize parallel spaced electrodes arranged in a longitudinal direction, between which a belt travels in the longitudinal direction that forms a continuous loop as it is driven by a pair of end rollers. A particle mixture is loaded into the belt between the electrodes where it is subjected to the strong electric field generated by the electrodes. The net result is that the positively charged particles subjected to the electric field move towards the negative electrode and the negatively charged particles move towards the positive electrode. The counter-current action of the moving belt segments sweep the electrodes in opposite directions and transport the constituents of the particle mixture to their respective discharge points on either end of the separator. Ultimately, each particle is transferred toward one end of the system by the counter-current moving belt that produces a certain degree of separation of the particle mixture.
The most established application to date for the triboelectric counter-current belt-type separator system is the separation of unburned carbon from coal fly ash. Worldwide, tremendous quantities of pulverized coal are burned in boilers to produce steam that powers turbines for the generation of electricity. In the boiler, the carbonaceous constituents in the coal are burned to release heat, and the non-carbonaceous material remains and is collected as fly ash. The ash content of normal coals vary, but typically comprise about 10% of the overall coal content. As a result, fly ash is produced at very high volumes throughout the industrialized world. Historically, one of the major outlets for coal fly ash has been as an additive in concrete products as a replacement for a portion of the cement. Furthermore, fly ash addition results in enhanced concrete strength and resistance to chemical attack, thereby turning a waste material to a valuable by-product. However, the presence of unburned carbon in fly ash has limited usage in concrete since implementation of The Clean Air Act of 1990 which required power plants to cut nitric oxide emissions through a variety of approaches including significant boiler modifications. These changes have resulted in elevated levels of unburned carbon in the fly ash that has rendered most materials unusable in concrete production without additional processing to remove unburned carbon. The counter-current belt-type separator system has proven to be one of the most cost-effective and reliable methods for processing fly ash for carbon removal. This technology typically produces a low carbon fly ash product, plus a fly ash stream that is enhanced in carbon content. As discussed, the low carbon product is ideally suited for use in ready mix concrete applications. On the other hand, the high carbon content fly ash is a valuable by-product due to its high fuel value which can be returned directly to the boiler for burning with the incoming coal. Alternatively, high carbon fly ash can also be used in other combustion applications such as a secondary fuel to cement kilns.